A great deal of inventive effort has been expended in providing suitable magnetic tapes for use as information storage media. Such tape media find use in audio systems, instrumentation systems, computer systems and video systems. They usually comprise a passive backing sheet and an active, i.e. magnetic, coating on the backing sheet.
During use, these tapes are subjected to rather large mechanical stresses caused by quick-accelerating winding devices, abrasion from static parts associated with reading apparatus, cleaning blades, contact with guides, etc. These stresses most commonly cause an eventual wear of the magnetic coating which may manifest itself by spurious errors appearing in the information stored on the tape. Such errors can be drop-out (the loss of information) or drop-in (the spurious addition of information).
One of the most fruitful areas for improving magnetic tape performance has been formulation of improved binder systems for carrying electromagnetic particles which form the information-carrying component of the tapes. The binder must adhere to a backing film, for example, poly(ethyleneterephthalate)-type substrates, and provide an abrasion-resistant, oil-resistant material of good physical properties.
It has been taught in the prior art that in order to obtain sufficient durability and/or performance for magnetic media applications, it is necessary to cross-link or cure the binder material. Cross-linking is defined as the attachment of two chains of polymer molecules by bridges composed of either an element, a group or a compound which joins certain atoms of the chains by primary chemical bonds. Generally, cross-linking is required to impart sufficient toughness (area under the stress/strain curve) or to impart an appropriate soft/hard segment ratio to the binder system for the particular application. It is recognized that cross-linking decreases elongation and increases strength and resistance to thermal, environmental, hydrolytic and chemical attack. It is the balance between tensile strength and elongation that is necessary to define or quantize the toughness required for a given media application. Presently, cross-linking is achieved through a combination of chemical and thermal operations.
Poly(vinylchloride) was one of the first polymers to be used in forming binders for tapes, but that material lacked a number of desirable physical properties and also tended to degrade in the presence of iron oxide particles embedded within the polymer matrix. More recently, tape development work has been done using a large number of polymers. Linear thermoplastic polyurethanes of the polyester and polyether types are probably the most common major components of magnetic media binder systems. Polyurethane materials were recognized as contributing desirable chemical resistance (as to cleaning solvents) to the tape systems and also imparted desirable physical properties, such as abrasion resistance, to the tape. B. F. Goodrich offers a number of polyurethanes sold under the trade name ESTANE, which are polyurethanes of either the polyester or polyether type. As taught in U.S. Pat. No. 4,020,227, prior art binders have included, besides polyurethanes, polyurethane/vinyl chloride-vinyl acetate-vinyl alcohol copolymer combinations, polyurethane/phenoxy combinations, polyurethane/vinylidene chloride-acrylonitrile copolymers, as well as vinyl chloride-vinyl acetate-vinyl alcohol copolymer/acrylonitrile butadiene copolymers.
The cross-linking carried out to produce acceptable binder components has been achieved through a combination of chemical and thermal operations. They primarily employ multifunctional isocyanates to impart an appropriate degree of cross-linking or toughness to achieve a proper soft/hard segment ratio consistent with the particular application intended for the given tape. Although the exact mechanism of cross-linking is unknown because of the complexity of the systems involved it is generally thought to occur via the reaction of "active hydrogen" groups of the binder or binder combination with the multifunctional isocyanates. Active hydrogen groups found both in polyurethanes and phenoxy resins, a common combination, have included such active moieties as --NHC(O)0--, --NHC(O)NH--, --OH, --COOH and --NH--. Reaction with these groups can result in cross-linking via formation of allophanate, biuret, urethane and urea linkages.
It is commonly known that isocyanates are extremely sensitive to water. The active isocyanate groups react with water molecules according to the following equations: EQU --NCO+H.sub.2 O.fwdarw.CO.sub.2 +--NH.sub.2 EQU --NH.sub.2 +--NCO.fwdarw. ##STR1## As a result of this, prior art processes have been constrained in the use of isocyanates and have turned to certain catalysts such as ferric acetylacetonate which favors allophanation rather than the reaction of the isocyanate with water. Furthermore, the use of isocyanate-containing resins for use as binders for magnetic tapes impose certain distinct process constraints. The coatings begin to harden spontaneously as cross-linking takes place and coating operations including the calendering of the binder layer surface must be accomplished during a narrow process window before a significant degree of cross-linking has occurred.
It is thus an object of the present invention to disclose a novel magnetic tape binder layer without the disadvantages outlined above.
It is a further object of the present invention to disclose a novel magnetic tape binder layer which is not highly dependent upon environmental constraints including the moisture content of the ambient surroundings.
It is yet another object of the present invention to disclose a novel magnetic binder layer which can be "hardened" at any time chosen by the process operator, even after all of the coating and calendering operations have taken place.
It is still another object of the present invention to disclose a novel magnetic tape binder layer which has both a hard and soft component so that the binder can be tailored to a specific end use, said components forming a semi-interpenetrating polymer network.
It is yet another object of the present invention to disclose a binder layer for magnetic tape which is hardened or cured solely through the use of electron beam irradiation.